Semiconductor epitaxial alloys are widely used in solid state applications. For example, defect-free III-V compound semiconductor alloys are responsible for the widespread application of a vast array of optoelectronic devices, including semiconductor lasers, light-emitting diodes, and solar cells. One of the key technological requirements for achieving high quality optoelectronic materials and devices is the epitaxial crystal growth. However, the versatility of semiconductor epitaxial growth is restricted by the limited number of single crystal substrates available in bulk form. Indeed, the limited number of single crystal substrates available in bulk form is currently an obstacle to the development of high quality semiconductor heterostructures as the variety of compositions and lattice parameters is restricted to those available as single crystal substrates in bulk form. Therefore, although the ability to modify, control, or tune, the lattice parameter, band gap or band offset of the substrate used in the epitaxial growth process would be ideal, it has been unattainable due to the limitations on available single crystal substrates. Such control, or tunability, would, for example, enable the fabrication of optoelectronic devices with unique characteristics, such as semiconductor lasers, thin film solar cells, and photonic crystals, among others. Unfortunately, the currently limited variety of available single crystal substrates is stifling such development.
In a coherent epitaxial growth process, the deposited material assumes the substrate crystal structure and lattice parameter, and the grown film is free of strain-relieving dislocations. The lattice parameter of the deposited film in the parallel direction (a∥) is equal to that of the substrate (asub), and the lattice parameter in the perpendicular direction (a⊥) is free to expand or contract due to the existence of a traction-free top surface and the Poisson effect, which preserves the material unit cell volume V (FIG. 1A). For a cubic symmetry, V=afilm3=a⊥−a∥2, where afilm is the lattice parameter of the relaxed film, the lattice mismatch between the film and the substrate induces a tetragonal distortion in the epitaxial film, defined as strain [ε∥=(a∥−afilm)/afilm]. If the film's natural lattice parameter (afilm) is smaller than that of the substrate (asub), it is tensile with respect to the substrate, and therefore the strain value is greater than zero (i.e., ε∥>0). Conversely, if the film's natural lattice parameter (afilm) is larger that that of the substrate (asub), it is compressed with respect to the substrate, and therefore the strain value is less than zero (i.e., ε∥<0). Any material that is lattice-mismatched to the substrate will be strained, causing an increase in the elastic energy of the system. If the elastic energy exceeds the energy associated with the introduction of defects such as dislocations, these defects can be introduced into the film to minimize the overall energy. However, the defects compromise the quality of the crystal, and consequently the device's performance.
Armed with this knowledge base, substantial research has been dedicated to the development of a defect-free crystalline substrate with a tunable lattice parameter for use as a template for epitaxial semiconductor growth. To date, however, none of these efforts have proven successful. For example, considerable effort has been devoted to the growth of graded layers in order to achieve a final lattice parameter distinct from that of the substrate initially used. However, this technique requires the growth of additional non-active layers, and depending on the growth conditions, may lead to dislocations.
Other prior efforts to make epitaxial growth substrates for Chemical Vapor Deposition (CVD) growth of materials with lattice constants different from those of single crystal wafers were based on a compliant substrate concept. This consisted of placing a thin layer of a crystal template on a planar support with a low viscosity interface between the two materials. However, the crystal template did not have the desired lattice constant. Instead, the layer was designed to be much thinner than the desired layers of CVD grown material. As the epitaxial growth process progressed, dislocations caused by the lattice mismatch strain between the template material and the growth material would be segregated into the thin template, allowing the growth material to attain its natural lattice constant with a low density of defects. The concept called for the low viscosity layer to allow the thin template to slip on the planar support and accommodate the mismatch strain. In reality, though, the interface between the crystal template and the planar support was not capable of allowing sufficient slip over the area of the entire compliant substrate. As such, this concept was not able to support the growth of large areas of single-crystal material.
In a different approach, high temperature annealing of a glass compound was used to enable strain relief of Si—Ge epitaxial films by means of elastic strain relief, but film buckling was consistently observed. Indeed, heat treatments over 400° C. cannot be performed on III-V semiconductor compound films without plastic deformation of the alloy films.
According to yet another approach, relaxation of InxGa1-xAs strained regions was achieved in selectively etched mesas that allowed very small areas of the film (300×300 μm2) to relax. However, although the film relaxes, it is trapped between other mesas, significantly limiting its applicability to device fabrication.
In an alternative approach, a free-standing, elastically-strained nanomembrane was achieved by growing a tri-layer of Si/SixGe1-x/Si, and relieving the tri-layer from the original substrate. In these laminate structures, elastic strain is partitioned among the layers of the tri-layer to achieve elastomechanical equilibrium. Although these structures can enable control of silicon band offsets, these templates consist of a combination of strained layers in a strain-balanced laminate, and none of the layers relax to their native lattice parameters nor is the laminate a single crystal substrate or a template for epitaxial growth.